Technical Field
The present invention relates to radiation monitoring devices that are used for radioactive emission control or radiation control such as in nuclear reactor facilities and spent nuclear fuel reprocessing plants.
Background Art
A radiation monitoring device used such as in nuclear reactor facilities and spent nuclear fuel reprocessing plants includes a multi-channel radiation monitor that is provided with a detector for detecting radiation to output detection signal pulses and a measurement unit for receiving the detection signal pulses to measure a count rate (the count rate is a value that is obtained by dividing a count value by a unit time—a second or a minute is generally used as the unit time—and is expressed by the unit of “cps” or “cpm”); and a test unit that is constituted with a test signal generator for inputting test pulses to an individual radiation monitor (using the test pulses allows for generation of a desired repetition frequency, and for check on response of the output count rate to the input signal and on accuracy of the response) and a test signal controller for controlling the repetition frequency of the test pulses generated by the test signal generator and for switching between inputs of the detection signal pulses and the test pulses in the radiation monitor. The test signal controller controls an input switching circuit in the measurement unit of the radiation monitor to switch the test pulse input from the detection signal pulse input, and performs an output-to-input response test for measuring a response of the count rate to the repetition frequency of the test pulses (the output-to-input response test here means a test of measuring linearity of output to input) and performs an alert test for checking the count rate at an alert activation point by inputting test pulses having a repetition frequency that varies across an alert setting level. Soundness of each radiation monitor is thereby checked (the soundness here means a condition that satisfies an accuracy specified by the spec).
Each radiation monitor converts, when necessary, a measured count rate to an engineering value such as a dose equivalent rate (dose equivalent increment per unit time; dose equivalent is used to evaluate an effect of radiation on radiation-exposed persons and expressed as “absorbed dose×quality factor”) to obtain a radiation dose. A high alert is set to a radiation level higher than the ordinary background level. For abnormality of a dose equivalent rate in a control area in such facilities and plants (it is confirmed that a dose equivalent rate is proportional to a count rate from measurements of dose equivalent rates and count rates at various points using a reference radiation source) or radioactivity in a process line in such plants (radioactivity is proportional to a count rate because double in radioactivity leads to double in radiation emission and to double in the count rate of detected pulses), the high alert is issued to give notice to operators and a necessary line isolation is automatically performed (for example, if there is an emission end port in the line, the line is shut-off by the high alert). Furthermore, a low alert is set to a radiation level lower than the ordinary background level. For detection signal loss due to failure of a radiation monitor or a low count rate of detection signal, the low alert is issued to give notice to operators.
Accuracy of measurement response and accuracy of the alert activation are checked by inputting test pulses to a computing part. A test signal controller changes the repetition frequency of the test pulses output from a test signal generator in a step-function or a ramp-function manner according to a test item (refer to JP 1110-260262 A). In addition, the alerts are blocked before the alert test is started so that the high alert and the low alert are not issued externally from the radiation monitoring device during the test period, and the block is released after the test is completed. These block operations are set manually. During the period of blocking the alert (the test time is shorter than the blocking period), an in-test warning is output.
Since a radiation dose measured by a radiation monitor fluctuates statistically (this is due to intrinsic atomic disintegration occurring at random; refer to The Japan Radioisotope Association, “Fundamentals of Radiation Handling” the Third Impression of the First Edit., Maruzen, December, 1996, pp. 299-302), the radiation monitor maintains a desired measurement accuracy by automatically controlling the time constant according to a count rate to keep the standard deviation constant. Moreover, the radiation monitor needs to perform a measurement covering a wide count-rate range from about 10 to 107 cpm (count per minute: the number of radiation counts per minute), and measures a count rate over the wide range without changing the range to eliminate discontinuity associated with range changing. Because a count rate responds with the time constant and the time constant is inversely proportional to the square of standard deviation and to the count rate (refer to “Fundamental of Radiation Protection”) and because it takes time to perform an output-to-input response test for a measurement range of every decade and for a high and a low alert tests, a contrivance is made to shorten the response time of a count rate to a test pulse input by combining, depending on each test item, input of test pulses having a repetition frequency changing in a step-function manner and input of test pulses having a repetition frequency varying in a ramp-function manner and by setting beforehand the amount of step change and the slope of ramp-function variation (refer to JP H10-260262 A).
However, a response test at a low count rate still needs to take a long time. For example, under the condition of a standard deviation of 2.6%, the lower limit of measurement range of 10 cpm, and the background of 50 cpm, the test takes approximately 40 minutes to approach to 10 cpm after a test pulse of 1 cpm is input in a step-function manner. Even inputting test pulses of 0 cpm to accelerate the response achieves no significant improvement effect.
In the conventional radiation monitoring device, as described above, test time is shortened by controlling, on the basis of a changing pattern of a step-function input and a ramp-function input set beforehand based on a target value, the repetition frequency of the input test pulses so as to immediately approach near the target value by the step-function input and then gradually approach toward the target value by the ramp-function input. However, since a count rate responds with the time constant and the time constant is inversely proportional to the square of the standard deviation and to the count rate (refer to “Fundamental of Radiation Protection”), execution of a test item particularly for a low count rate will fall into a situation of simply waiting for lowering of the count rate with a time constant, raising a problem that the test inevitably takes a long time. Moreover, almost all channels (the channels here mean respective ones of a plurality of radiation monitors in a radiation monitoring device) are in the condition of continuous measurement during operation of such a plant, and there is inevitably a channel that should not be in a condition of missing measurement for a long time because of need for continuous measurement even during a regular inspection, requiring further improvement for demand for test time reduction to minimize the missing measurement.
Furthermore, when a test item is altered midway during a scheduled test, a test input pattern needs to be reset and additionally computed in accordance with the altered test item and an altered target count rate, raising a problem in quick responsiveness. That is, when an unscheduled alteration happens, the test time increases drastically and the test may not be completed within the scheduled time in some cases.
The present invention is made to resolve the above problems and aimed at providing a radiation monitoring device that is capable of significantly reducing a test time and flexibly altering a test item when soundness of a device needs to be checked urgently.